Photography became popular because of the 35mm format. The actual size of the frame is 36mm by 24mm so I'm not sure why it’s called 35mm. The point here is that SLR digital cameras use the same lenses that a 35mm film camera does. Consumer models use smaller lenses and quote the focal length in 35mm equivalents. Keep in mind there’s a difference between 35mm as a format, and 35mm as a focal length, which also determines the field of view.
The use of the 35mm standard made it convenient for photographers who gravitated from film to digital. However, most digital SLRs have smaller sensors, making them more affordable. However, this also causes a cropping of the image projected by the lens and results in a telephoto effect. My 10D has a crop factor of 1.6; therefore my 50 mm macro lens is equivalent to an 80mm lens on a 35mm film camera. Of course we are now seeing full frame digital SLRs meaning that the sensor is the same size as the 35mm film frame, so these cameras do not crop the image. The least expensive is the Canon 5D which as of this date sells for $3000, no lenses included.
So most digital SLRs have a crop factor, and certainly all of those within the price range of the average enthusiast do. Now there are different ways of looking at this. On one hand my 7-300mm zoom suddenly taps out 480mm. That’s a nice long shot. But the 20mm I just bought is effectively a 32mm, which drops the field of view from almost 90 degrees to maybe 60. That’s still wide angle, but it also cost me another $200 to get it.
Consumer models have fixed lenses. All that I know of have zoom lenses, some that look like an SLR and provide as much as a 12x magnification. The focal length of the lens is quoted in a 35mm equivalent, but the lens itself is actually many times smaller. The reason for this miniaturization is that the sensors are very small. My Canon G1, for instance, has a sensor that measures 7.18mm x 5.32mm, or about one-fifth the size of a 35mm film frame.
To fit the lens to the sensor, the lens had to be made smaller. So the 7mm to 21mm zoom lens is quoted as a 35mm to 105mm equivalent because the field of view is the same through the lens as those focal lengths on a 35mm film camera.
The reason for bringing this up involves the effect on depth of field. Normally when taking a photograph some consideration is given to the depth of field because you always want the entire subject in focus, and just the same there are times when you’d like the background to be out of focus (this is known as bokeh, a Japanese word). The problem that I and others had with the early digitals was that there was no bokeh even with the lens wide open. And there was a bit of wonder when at f4 we were getting sharp pictures of objects some considerable distance away.
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